Building a 1/4 Wave Ground Plane Antenna for 2 Meters

What do you do when you have a spare SO-239, and some metal rods laying around? Build a 2m ground plane antenna! Why not, I think its about time I build one of the most popular antennas known to hams! . Why in the world would I want to build a 0dB gain antenna? Well just for kicks for starters, but there are also specific applications where a 1/4 Wave GP antenna may be a better option than a 5/8 wave. (or vice versa).

Here’s the antenna modeling for both:

As you can see, the 5/8 wave has about 3 dB of gain as it shoots more of your signal out towards the horizon but has sharper & deeper nulls than the GP. Well if its further, then it must be better! Not exactly… You may be missing out on signals that the 1/4 wave could be picking up! So there are advantages/disadvantages to both, really depends on what application your going to be using it for. You really need to experiment with both types over an extended period of time to get an accurate picture of what would work better. Everyone’s shack is different from the next, and unless your on completely flat ground for miles and miles, the choice may not be so clear!

My primary VHF/UHF antenna is a Tram 1480, which has 2 – 5/8λ elements for 2 meters & 4 – 5/8λ elements for 70cm. These stacked ‘in-phase’ elements really pancake your signal pattern on the horizon, so I’m very interested to see how a normal GP antenna will function compared to all the others I’ve used up until now!

So we’ll make the radials just a tad longer, about 20 3/16″ to make up for the loop we’ll make to attach to the SO-239. The ARRL antenna book has a few different options for this type of antenna. I opted to try using a copper plate under the radials (they suggested aluminum) I had laying around. I noticed as soon as I started attaching the radials to the SO-239 that this wasn’t going to be that sturdy, so I wanted to add the plate for added stability and to give it some rigidity. This did add some weight to the antenna though, so it needed 1 1/2″O.D. pvc pipe to hold it up sturdy.

For mounting purposes, you can get really creative with these things, or you can simply stick this on top of a piece of PVC pipe, and run your coax out through a “T” connection before the pipe hits the ground to protect your coax (or just cut 3/4″ hole in the PVC).

The metal rods I used were just a little too thick for the SO-239, so I had to grind down the main feed pole down to a point, no grinder here, so used a dremel, worked great! I also had to use self-tapping screws as I never have the hardware to install SO-239’s properly:

Bending the end of the radials where they attach to the SO-239 was pretty tough, pretty sure this is stainless steel, and even using needle nose pliers wouldn’t work, I had to get really creative in making those loops at the end, to include using a hammer! I’m sure if you were using a coat hanger, which has much softer metal to work with, it’d be easier. So I tinned the end of the vertical monopole and soldered it to the SO-239, but noticed that even after being soldered that the connection point was not strong enough for my liking, a bird could fly into this thing and break it off, lol! Really weak! So I gooped a BUNCH of epoxy on top of the SO-239 mount, probably way too much, but at least its weather protected now!

I put the antenna at the edge of the table to bend the radials down to 45 degrees (estimated here!):

First confirmed contact on a random CQ on 146.52 was KE5AST – using only 8 watts, 50 feet of RG213U & my Baofeng GT3TP with an SO-239 to SMA adapter! So definitely not the best setup!

Nothing to rave about, but 14 miles on an HT? That’s not bad! Scott did have to bump his power up to 40 watts so I could hear him, but he said I was completely readable & full quieting! SWR was virtually flat all across the 2m band using my bridge meter.

GP’s also have a low footprint, so if your in an HOA situation this might work a bit more stealthy than some of the commercially available options out there due to those types having a large fiberglass mast that’s quite visible. A simple antenna like this made out of thin elements, would be harder to see from a distance.

If I did it all over again, I’d probably try it without the plate to reduce weight. I don’t think the plate adds anything that the radials don’t other than providing a more stable surface to attached stuff to, and perhaps even attach a mounting bracket of some kind perhaps?

Want to thank YouTube User Wizardsof12 for the inspiration & a great YouTube video on building one of these! Checkout his video:

Overall, I’m quite pleased with the results & will most likely use this for field days, & possibly use it as a dedicated APRS antenna.

I use brass rod for all the elements. Fairly easy to put the loops on the end but certainly not floppy.
Bought mine at Ace Hardware.

Andrew VK1AD

16 September 2016 at
8:14 PM

Nice job and always handy for local portable communications. If you fancy experimenting with a little more gain, I recommend constructing a 2m SlimJim from 450 ohm feedline, brilliant homebrew vertical antenna with a low radiation angle.
73 Andrew VK1AD.

Johnny

16 September 2016 at
8:32 PM

Thanks Andrew! I’ve still yet to even touch ladder line! I used a j pole once but it had 3:1 SWR on UHF! I’ll keep that in my ‘to-do’ file!